1. Technical Field
The present invention relates to a light transmission system and an electronic device equipped with the same.
2. Related Art
In recent years, increase in the data transmission speed between an LCD (Liquid Crystal Display) and an application processor is desired with higher definition of the LCD of a portable telephone. Reduction in the number of wirings for data transmission is also desired with thinning of the portable telephone. Based on such background, serial transmission is starting to be widely used in place of the conventional parallel transmission as a method of transmitting data between the LCD and the application processor. Currently, data signals and clock signals are both transmitted in a differential transmission method in the serial transmission that is being most widely used. In such serial transmission, unnecessary radiation (EMI; Electromagnetic Interference) is becoming more and more serious in recent years. Thus, the clock signals are transmitted at low speed to reduce the EMI. In such transmission method, the there is EMI from high speed data signal.
A method of connecting the LCD and the application processor with a light guide and the like, and transmitting the data signal as an optical signal is being considered.
The light guide has a double structure of a center core, which is called a core, and a capsule, which is called a clad, where the index of refraction of the core is higher than that of the clad. The optical signal entered to the core is propagated by being totally reflected repeatedly inside the core.
Through the use of the light transmission module, large capacity data signals can be transmitted at high speed from a main control board to an application circuit board mounted in the portable telephone. The light transmission module thus excels as a data transmission module.
When transmitting the data signal as an optical signal and transmitting the clock signal as an electrical signal, a large delay is generated between signals at a light receiving unit 3. Thus, there is skew (temporal shift) adjustment between the data signal and the clock signal compared to a module of electrically transmitting both the data signal and the clock signal.
A delay amount of the clock signal is accumulated in a memory on a transmission device side, and the clock signal is transmitted with delayed by the delay amount accumulated in the memory to adjust the delay in Japanese Unexamined Patent Publication No. 2007-124130 (date of publication: May 17, 2007). FIG. 22 is a block diagram showing a configuration of a transmission and reception device described in Japanese Unexamined Patent Publication No. 2007-124130.
As shown in the figure, a transmission device 200 includes a transmitting unit 214, a drive unit 216, a laser diode 218, a delay unit 222, and a delay information holding unit 224. The transmitting unit 214 transmits the data signal and the clock signal to each transmission path of an optical fiber 232 and a clock transmission line 234. The drive unit 216 supplies drive current to the laser diode 218. The delay unit 222 delays the clock signal by a time corresponding to a phase difference signal transmitted from a reception side. The delay information holding unit 224 holds data showing the delay time to be delayed by the delay unit 222. In the transmission and reception device of Japanese Unexamined Patent Publication No. 2007-124130, the delay unit 222 delays the clock signal based on the data, which shows the delay time calculated from a wiring length, accumulated in a memory 240 of the delay information holding unit 224 to adjust the delay between the data signal and the clock signal.